1. Field of the Invention
This invention relates to an apparatus and a process for sputtering which is useful in the formation of integrated circuit structures on semiconductor wafers. More particularly, this invention relates to an apparatus and a process for increasing the uniformity of the sputtering rate of sputtering apparatus useful in the formation of integrated circuit structures on semiconductor wafers.
2. Description of the Related Art
In the deposition of materials onto a substrate by sputtering from a target, the sputtered atoms from the target are scattered in various directions, including back to the target where they may then be resputtered by the ions, e.g., argon ions, impinging on the target. However, sputtered atoms which are scattered back to redeposit on the target do not adhere well to the target. Therefore, if such atoms are not resputtered, they may build up as a poorly bonded mass of materials which eventually flake off, giving rise to undesirable particle formation in the deposition chamber.
Unfortunately, the sputtering rate is not uniform across the entire face of the target, in part due to the presence of a grounded shield adjacent the periphery of the target which serves to protect the walls of the deposition chamber from deposition of the sputtered atoms. Thus, while the scattering and redeposition of sputtered atoms in the chamber is fairly uniform, sputtering of the redeposited atoms (i.e., resputtering) is not uniform, with less of the redeposited atoms being resputtered adjacent the periphery of the target (which is usually circular in shape).
When a single material such as a metal, e.g., titanium, is being sputtered, the sputtering rate is usually sufficient that the above-described redeposition does not create a problem. That is, the resputtering rate is still sufficient to inhibit excessive build up of redeposited metal at the edges of the target.
However, when reactive sputtering is carried out in the deposition reactor, wherein the sputtered metal atoms react with a gas in the chamber to form a metal compound on the surface where the metal atoms deposit, such a metal compound may have a lower sputter rate than the metal itself. When the sputtered metal atoms redeposits on the target surface and react with the reactive gas to form the metal compound, this metal compound is sometimes resputtered at an insufficient rate to prevent or inhibit the undesirable build up of a poorly adherent mass, thus giving rise to the above described undesirable formation of particles in the chamber as the metal compound flakes off the target surfaces where it has formed.
This redeposition and particle formation problem has been found to be of particular importance in the reactive sputter deposition of titanium nitride on a substrate surface, e.g., the surface of a semiconductor wafer. The sputtered titanium atoms, when scattered and redeposited onto the target surface, react with the nitrogen gas in the chamber to form titanium nitride on the target surface. This titanium nitride, in turn, sputters at a lower rate than does titanium metal. When the titanium nitride forms at the periphery of the target surface, the combination of redeposition and a lowered resputtering rate results in a gradual build up of poorly adherent titanium nitride at the periphery or edge of the target, thus giving rise to particle formation as the material flakes off the target.
It would, therefore, be desirable to provide a sputtering apparatus and a process capable of reducing the amount of such redeposition of the sputtered material at the edge of the target and/or to increase the sputtering rate, at the edges of the target, of the redeposited material; particularly when reactive sputtering is carried out in the apparatus and the redeposited material forms a compound by reaction of the initially redeposited material with a reactive gas present in the sputter deposition chamber.